1. Field of the Invention
The present invention relates to a method of producing an oxidation catalyst for cleaning exhaust gas, which oxidizes particulates, polycyclic aromatic hydrocarbons and the like contained in the exhaust gas discharged from internal-combustion engines to clean the exhaust gas.
2. Description of the Related Art
In order to clean the exhaust gas discharged from internal-combustion engines such as a motorcar engine, an oxidation catalyst for cleaning exhaust gas in which a noble metal as a catalyst is supported on a heat-resistant carrier such as alumina, and ceria or the like is further mixed to promote oxidation reaction has been conventionally used. The conventional oxidation catalyst for cleaning exhaust gas can oxidize volatile organic compounds (VOCs) having low boiling points contained in the above exhaust gas, but cannot achieve sufficient function for oxidizing the particulates and polycyclic aromatic hydrocarbons.
This is due to that the particulates and polycyclic aromatic hydrocarbons have high boiling points and are chemically more stable than the VOCs. In order to oxidize particulates, polycyclic aromatic hydrocarbons, and the like contained in the exhaust gas, it is therefore conceivable to use a composite oxide containing two kinds of metal elements as an oxidation catalyst capable of giving an atmosphere of stronger oxidation reaction.
As the composite oxide, there is known, for example, a compound having a perovskite structure, represented by the chemical formula of AB1-xCxO3 wherein A is at least one of lanthanum, strontium, cerium, barium, and calcium; B is at least one of cobalt, iron, nickel, chromium, manganese, and magnesium; and C is at least one of platinum and palladium (see, for example, Japanese Patent Laid-Open No. 2003-334443). The compound having a perovskite structure is considered capable of decreasing the combustion starting temperature of particulates because the platinum or palladium is activated.
As the composite oxide, there is also known, for example, a cerium-zirconium composite oxide (see Japanese Patent Laid-Open No. 7-116519). The cerium-zirconium composite oxide has an oxygen storage capacity, and is considered to release the oxygen when the cerium atom produces a valence change from tetravalency to trivalency in a reducing atmosphere.
However, the conventional composite oxides are not sufficient in the function of oxidizing particulates, polycyclic aromatic hydrocarbons, and the like contained in the above-described exhaust gas, and desired to be further improved.